The present invention is in the field of time delay devices, such as those that may be used for the control of phased-array radars, communication systems, or correlators.
This invention relates to apparatus for producing true-time delay devices, such as those useful in the control of phased array radars. It is desirable to use a system that produces signals to control the timing of the emission of each of a plurality of electromagnetic radiation beams, delaying each of them in time by some time increment. The delay in each signal should be capable of being controlled independently of the other signals.
Phased array radars have the advantage that the radar beams can be steered electronically by changing the phase or timing of the signal radiated by the individual elements of the array. Often, this is accomplished by controlling the phase of the signals applied to the array elements. This procedure introduces undesirable squint if very short pulses or broad bandwidths are required. True time delay offers a scheme for controlling the elements without squint even with broadband signals.
Electronically implementing the true time delays is generally impractical because of the need for many long lengths of strip line, waveguides, or coaxial cable, which are expensive, bulky, and temperature sensitive. Because long paths are comparatively easy to obtain optically, photonic systems present a means of obtaining the beam agility of array systems combined with wide bandwidth. Approaches to true time delay tend to fall into two categories: those using fibers and those using long free-space paths. Some fiber approaches use multiple optical switches or broadcast the light over all possible paths at once. Wavelength-division-multiplexing schemes have recently been developed by use of fiber Bragg gratings. Free-space systems have also used multiple optical switches for switching the beams between sequential optical paths. These optical switches are usually liquid-crystal based.
It is therefore an object of the current invention to create a device for optically generating true time delays that is inexpensive, is compact in design, and is sufficiently temperature insensitive.
Although described with respect to the field of phased-array radars, it will be appreciated that similar advantages of optically producing true-time delays, as well as other advantages, may obtain in other applications of the present invention. Such advantages may become apparent to one of ordinary skill in the art in light of the present disclosure or through practice of the invention.
The present invention includes time delay devices and time delay systems. The invention also includes machines and instruments using those aspects of the invention. The invention may also be used to upgrade, repair, or retrofit existing machines or instruments, using methods and components known in the art.
The present invention comprises a true time device that falls into the free-space category but uses a multiple-pass optical cell with refocusing mirrors that has the advantage of avoiding beam-spreading problems. This approach differs from previous free-space approaches in that it uses only one optical switch or spatial light modulator instead of one or more switches for each bit. In this approach the microwave signal for each antenna element may be modulated onto an optical beam. After the individual optical beams are delayed by the desired amount of time, the signals may then be down-converted to microwave signals for further processing. This process may be used in either the transmit or the. receive mode of the phased array radar.
In broadest terms, the device for producing optically-controlled incremental time delays of the present invention comprises: (1) an input device selected from the group comprising light sources adapted to generate individual light beams or arrays of light beams from one or several directions, (2) an adjustable input mirror capable of reflecting light from the input device in different directions, (3) a set of optical elements selected from the group consisting of mirrors, lenses, filters and prisms placed in a configuration so as to define a multitude of light paths for each light beam from the input device reflected by the adjustable input mirror, (4) at least one refocusing element to restrict the divergence of a light beam diverted through at least one of the light paths, (5) a spatial light modulator adapted to select a path from among the light paths for each pass through the set of optical elements of an individual light beam from the input device, (6) an output mirror adapted to reflect the light beams emerging from the set of optical elements, and (7) a receiving device capable of responding to the delays in the light beams reflected by the output mirror.
The device for producing optically-controlled incremental time delays of the present invention may also include at least one system of optical transmission lines or waveguides wherein the lengths of the light paths may be varied in a confined space consisting of a subset of the optical elements. The spatial light modulator may consist of a polarizing spatial light modulator which changes the polarization of individual light beams directed to the spatial light modulator. The use of a polarizing spatial light modulator then may require the addition of a beam-splitting device that can direct light beams through the system of optical elements in multiple directions depending on the polarization of the light beams after passing through the polarizing spatial light modulator. The device may alternatively include a micromechanical or deformable mirror device spatial light modulator capable of reflecting the individual light beams in multiple directions, thereby determining the optical path.
The light sources adapted to generate individual light beams or arrays of light beams from one or several directions may include such devices as lasers, arc lamps, and light emitting diodes. The receiving devices capable of responding to the delays may include devices such as photodetectors, pin diodes, photodiodes, and interferometers.